A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such elevated temperatures may contribute to increased nitrogen-oxide (NOX) emissions and may accelerate materials ageing in the engine system, including exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is a popular strategy for combating these effects. EGR works by delivering exhaust gas having reduced oxygen content to the intake, which results in lower combustion and exhaust temperatures. In particular, EGR variants that deliver cooled EGR are desirable because they can supply a relatively large flow of exhaust gas to the intake. However, cooled EGR is liable to cause transient control difficulties in boosted engine systems, particularly in combination with spark-ignition. For instance, throttle closure in a system configured for cooled EGR may trap a significant volume of compressed, EGR-diluted air upstream of the throttle. Such trapping may occur on transitioning from high to low engine load, for example. Under low-load, closed-throttle conditions, however, the engine may require fresh air to sustain combustion. Opening a compressor by-pass valve at this time provides a partial, but incomplete remedy for the problem, as the EGR-diluted air remains upstream of the throttle, albeit at a lower absolute pressure.
Excess intake pressure may be vented to other locations of an engine system besides the compressor inlet. Accordingly, U.S. Pat. No. 5,724,813 provides a compressor bypass valve that vents pressurized intake air upstream of a throttle to an exhaust conduit immediately upstream or immediately downstream of a turbocharger turbine. However, this system is not suitable for improving transient control in a modern engine system equipped with cooled LP EGR, as it lacks both an EGR system and an exhaust-aftertreatment system.
The inventors herein have recognized that improved transient control in an LP EGR equipped engine system can be achieved by judiciously configuring compressed air release with respect to EGR and exhaust-aftertreatment components. In one embodiment, therefore, a method for controlling an engine with exhaust gas recirculation is provided. The method comprises directing EGR from exhaust downstream of an exhaust-aftertreatment catalyst to mix with fresh air upstream of an intake compressor and upstream of a throttle valve, and, in response to increased throttling by the throttle valve, discharging the mixed EGR and fresh air from downstream of the intake compressor to the exhaust downstream of the exhaust-aftertreatment catalyst.
In this manner, a significant amount of pressurized, EGR-diluted air from behind the throttle can be quickly eliminated, thereby alleviating at least some transient-control difficulties associated with cooled LP EGR. Further, the disclosed methods avoid release of untreated exhaust into the ambient, and further avoid damage to compressor components due to soot entrained in recirculated exhaust gasses.